    :O "#FF0"
    :T "#A0F"})
 
+(def cell-filled? (complement zero?))
+
 ;;------------------------------------------------------------
 ;; Pure Functions operating on a board.
 ;;------------------------------------------------------------

          (swap! state assoc-in [:position 1] ny)
          (do
            (swap! state assoc :board (write-piece-to-board piece x y board))
            (spawn-piece!))))
      (recur))))


;; Board.
;;------------------------------------------------------------

(def empty-row (vec (repeat 10 0)))
(def empty-board (vec (repeat 22 empty-row)))

; The starting position of all pieces.
(def start-position [4 2])
